1. Field of the Invention
The invention relates to a control system for an automatic transmission, for changing a shift map by comparing an acceleration from an input torque and an acceleration from a vehicle speed to estimate a running resistance and, more particularly, to a control system for an automatic transmission for changing the shift map accurately even when the engine is cold.
2. Related Art
In the control system for an automatic transmission in the prior art, a gear change is effected according to predetermined shift characteristics. However, these shift characteristics are determined on the basis of an ordinary running status so that they cannot always satisfy the driver for the road gradient situations of uphill or downhill roads, for example. In order to achieve a gear change matching the road gradient situations of uphill or downhill roads, therefore, there has been proposed the so-called "hill control" (as disclosed in Japanese Patent Laid-Open No. 45160/1986) for changing the shift characteristics according to the road gradient situation, as determined from a running resistance, by estimating the running resistance from the running status of a vehicle as including an input torque coming from the engine.
In the prior art described above, however, the engine output drops at cold or overheated conditions of the engine, for example. As a result, a decrease in accuracy occurs that can result in a misjudgment that an uphill road is being followed even though the vehicle actually is running on a flat road. This may result in a misapplication of the hill control.
In order to solve this problem, there has been proposed a technique (as disclosed in Japanese Patent Laid-Open No. 266154/1990) for selecting a map to calculate a running resistance estimated value in accordance with an engine water temperature and an A/T oil temperature.
When the running resistance estimated value calculating map is to be selected, as in the prior art, according to the engine water temperature or the A/T oil temperature, a memory having a larger capacity is necessary for storing the maps according to the temperatures. When the aforementioned hill control is added to the existing electronic control unit (ECU), for example, the excessive data required for the hill control may make the memory short of capacity, and the memory may have to be replaced by another having a larger capacity. Moreover, this change in the memory makes it necessary to change the circuit design for the entire ECU. Further, in order to prepare the data for the running resistance estimated value computing maps, there are required a number of steps for calibrations.